Hydroxy-containing ethylenically unsaturated polyesters prepared from glycidyl polyethers of polyhydric phenols and ethylenically unsaturated monocarboxylic acids tend toward premature gelation both during the esterification step and during subsequent storage of the polyesters. Such premature gelation can be significantly reduced by the addition of compounds such as dialkylhydroxylamines and the like. See, for example, U.S. Pat. No. 3,408,422, issued Oct. 29, 1968. The use of hydroquinone as a gelation inhibitor in vinyl ester formulations is well-known and universally employed. In fact, the hydroquinone is frequently introduced into the vinyl ester through the ethylenically unsaturated monocarboxylic acid. In other words, the acrylic or methacrylic acid used to prepare the vinyl esters contain hydroquinone as an inhibitor. Under certain conditions, more hydroquinone is added during the esterification step or to the finished vinyl ester and styrene blend. As noted hereinbefore, other stabilizers may also, and usually are, used in combination with the hydroquinone, such as maleic anhydride, diethylhydroxylamine, paratoluenesulfonic acid, triethyl phosphite, and the like.
It is not uncommon to produce vinyl ester compositions containing up to 600 parts per million of hydroquinone in the vinyl ester or blend thereof. It is known that while the presence of hydroquinone stabilizes the vinyl ester, the resulting vinyl ester possesses color which is undersireable for many applications. It is believed that the hydroquinone oxidizes to benzoquinone and oxidized products of benzoquinone, which products impart the undesirable color to the vinyl esters and styrene blends thereof.
It has now been found that the color of the vinyl esters can be significantly improved by the addition of the so-called "heat" stabilizers, which are well-known antioxidants for polyolefins such as polypropylene. See, for example, U.S. Pat. No. 3,502,613, issued Mar. 24, 1970. It has further been found that the hydroquinone can be reduced to levels of below about 400 ppm and generally below 200 ppm.